Error: string index out of range error

I tried to use socketIO_client to recv data from my server.And it raise "string index out of range error" when the opcode is 8.The opcode is definded in websocket protocol.
Error message:

> Traceback(most recent call last):
> File "/usr/lib64/python2.7/threading.py", line 804, in __bootstrap_inner
> self.run()
> File "/home/study/socketio/socketio_client.py", line 115, in run
> socketIO.wait()
> File "/usr/lib/python2.7/site-packages/socketIO_client/__init__.py", line 251, in wait
> self._process_packets()
> File "/usr/lib/python2.7/site-packages/socketIO_client/__init__.py", line 276, in _process_packets
> for engineIO_packet in self._transport.recv_packet():
> File "/usr/lib/python2.7/site-packages/socketIO_client/transports.py", line 157, in recv_packet
> packet_text)
> File "/usr/lib/python2.7/site-packages/socketIO_client/parsers.py", line 96, in parse_packet_text
> packet_type = int(get_character(packet_text, 0))
> File "/usr/lib/python2.7/site-packages/socketIO_client/symmetries.py", line 26, in get_character
> return chr(get_byte(x, index))
> File "/usr/lib/python2.7/site-packages/socketIO_client/symmetries.py", line 30, in get_byte
> return six.indexbytes(x, index)
> File "/usr/lib/python2.7/site-packages/six.py", line 655, in indexbytes
> return ord(buf[i])
> IndexError: string index out of range
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